1. Field of the Invention
The present invention relates to the field of multicore optical fibers.
Even more specifically, the object of the present invention is to provide means of measuring optical coupling between the cores of a multicore fiber.
2. Description of the Prior Art
The concept of multicore fibers appeared in 1978, for multimode fiber applications. It was subsequently adopted in about 1986 for single-mode applications.
Further, in this kind of application, it would seem desirable to be able to accurately measure the coupled power, and the variation in coupled power, between the cores as a function of the length of the fiber.
Such a measurement would be of fundamental importance for specifications of transmission systems based on these multicore fibers. This is because it is essential to ensure that, in any installation, care is taken to keep the upper limits of the stray signal due to the inter-core coupling below an acceptable threshold.
However, to the inventors"" knowledge, no system which performs well and is reliable has yet been proposed for this purpose.
It is generally accepted that the crosstalk is a decreasing function of the inter-core distance h and an increasing function of the mode diameter 2wo, it is also an increasing function of the fiber length when the ratio h/2wo greater than  greater than 1 and coupled-mode theory cannot then be applied. Until now, there has been no way of carrying out accurate analysis and measurement on a multicore fiber.
Another important object of the present invention is to provide means which permit reliable measurement without the need to section the fiber being tested.
The above objects are achieved in the context of the present invention using a system comprising transmitter means capable of transmitting a train of pulses of controlled frequency and width into one end of a core of a multicore fiber, and detection means capable of detecting the signal received at the same end of at least one other adjacent core of the multicore fiber, with a controlled time window close to the transmission pulse duration and with a delay between 0 and the transmission period.
According to another advantageous characteristic of the present invention, the processing means comprise means capable of adding, to the signal obtained at the detection means, a signal representative of the power lost by attenuation along the core placed in line with the detection means, in order to obtain the crosstalk.
According to yet another advantageous characteristic of the present invention, the processing means also comprise means capable of adding, to the crosstalk signal, a signal representative of the power lost by attenuation along the core placed in line with the transmitter means, in order to obtain the far-end crosstalk.